Japanese Patent Laid-Open No. 3-207587 discloses a laser welding apparatus that evaluates the quality of a welded part by means of light emanating from the molten metal of the welded part. Referring to FIG. 9, the laser welding apparatus will be described below.
As illustrated in FIG. 9, the laser welding apparatus welds a workpiece 1 by irradiating the workpiece 1 with a laser beam 3. The laser beam 3 is continuously emitted with a constant intensity from a laser oscillator 11. The laser beam 3 emitted from the laser oscillator 11 is transmitted to an optical condenser system 13 through a laser-beam transmission optical system 12. The optical condenser system 13 focuses the laser beam 3 onto a welded part 2 and the workpiece 1 is welded by the focused laser beam 3. The molten metal of the welded part 2 emits light during the welding of the workpiece 1. The light emanating from the molten metal is focused by the optical condenser system 13 and then is transmitted to an interference filter 15 through a monitor-light transmission optical system 14. The interference filter 15 selects light having a wavelength component identical to that of the light emanating from the molten metal, out of light transmitted through the monitor-light transmission optical system 14. The light selected by the interference filter 15 is received by a photodiode 16. When receiving the light, the photodiode 16 generates a signal corresponding to the intensity of the received light. The signal generated by the photodiode 16 is inputted to a computer 19 through an amplifier 17 and an A/D converter 18. A correlation between an emission intensity of the molten metal of the welded part 2 and a penetration depth of the welded part 2 is stored beforehand in the computer 19. The computer 19 matches the signal inputted from the A/D converter 18 with the stored correlation, thereby deriving the penetration depth of the welded part 2. The computer 19 then evaluates the quality of the welded part 2 according to the derived penetration depth.